Revisiting Ventilation

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Revisiting Ventilation

An updated overview of residential ventilation systems

Posted on Nov 17 2017 by Martin Holladay

My comprehensive article on residential ventilation systems, “Designing a Good Ventilation System,” was published back in 2009. A few things have changed in the last eight years, so it’s time to revisit the topic.

Code requirements

Most building scientists aren’t willing to provide a simple answer to the question, “At what point is a home so tight that the home requires a mechanical ventilation system?” A typical answer is, “It depends — but unless your house is very leaky, it’s better to err on the side of caution and install a mechanical ventilation system.”

Building codes aren’t so vague, however. According to the 2012 and 2013 versions of the International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.), any new home with a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. result of less than 5.0 ach50 is required to have a whole-house ventilation system. This code requirement can be found in Chapter 3, section R303.4, and in Chapter 15, section M1507.1 of the IRC.

Since the new IRC code requires homes in all zones except Zones 1 and 2 to achieve an airtightness result of no more than 3 ach50, the code effectively mandates a whole-house mechanical ventilation system for homes in Zones 3 through 8. If you live in Zones 1 or 2, and if your blower door test came in at less than 5.0 ach50, your home is also required to have a whole house ventilation system.

The bottom line: If you’re getting your advice from, you’ll be building a tight house — so your house needs a mechanical ventilation system.

The building code is vague concerning the details of a mechanical ventilation system; it doesn’t really tell builders what type of equipment is needed to comply with the code. The code is specific, however, about ventilation rates. Here are the minimum airflow rate requirements according to the 2013 IRC, section M1507.1:

These code-mandated ventilation rates are consistent with the formula promulgated by ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant., the most common residential ventilation standard, which sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area. (Most codes and standards assume that the number of occupants is equal to the number of bedrooms plus 1.)

Not all experts agree on ventilation rate recommendations, however. For more information on this issue, see these three articles:

Five different approaches

Most builders choose from a limited number of ventilation options. The most common options are listed below.

An HRV or an ERV. Almost everyone agrees that the best way to ventilate a home is with a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV). In all cases, dedicated ventilation ductwork makes more sense than trying to hook up an HRV or an ERV to the ductwork used by your heating or cooling system.

An HRV or an ERV will have the lowest operating cost of any ventilation system (because during the winter, these units can “recover” some of the heat that would otherwise be exhausted, and during the summer, some of the heat in the incoming supply air stream can be transferred to the exhaust air — assuming your house is air-conditioned).

Unfortunately, compared to other ventilation options, HRVs and ERVs have a relatively high installation cost (generally between $4,000 and $10,000).

For more information on these ventilation systems, see the following articles:

A central-fan-integrated supply ventilation system. This type of ventilation system only works in a house with a forced-air heating and cooling system — so if you heat and cool with minisplits, this option is out.

A key element of a central-fan-integrated supply ventilation system is a fresh air duct that brings outdoor air to the return plenum of the home’s furnace or air handler. Once this fresh air duct is installed, some ignorant HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors think they’re done. They’re not. To prevent overventilation and underventilation, a contractor needs to install two more pieces of equipment: a motorized damper in the fresh air duct, and an AirCycler control (also known as a FanCycler control) connected to the motorized damper and to the furnace’s blower motor. The contractor also needs to perform an additional essential task: commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. the ventilation system by measuring and adjusting the airflow through the fresh air duct when the furnace fan is operating.

A central-fan-integrated supply ventilation has at least two virtues: It costs less than an HRV or ERV, and it provides better distribution of fresh ventilation air than an exhaust-only ventilation system. But few HVAC contractors are capable of measuring airflow rates or commissioning a central-fan-integrated supply ventilation system — one reason why these systems aren’t more common.

One last point: If you intend to install a central-fan-integrated supply ventilation system, make sure that your furnace has an ECM blower (that is, a blower equipped with an electronically commutated motor). An ECM blower is energy-efficient. If your furnace lacks an ECM blower, you'll have sky-high electricity bills with this type of ventilation system. (Some inefficient furnace blowers draw as much as 800 watts.)

For more information on central-fan-integrated supply ventilation systems, see “Designing a Good Ventilation System.”

Lunos-type fans. Lunos e2 fans come in pairs. Each fan is mounted in a 6-inch-diameter hole drilled through an exterior wall. Each cylindrical fan unit includes a perforated ceramic core. Every 70 seconds, the fan automatically reverses the direction of the airflow through the ceramic core. The effect of this cycling — supply fan / exhaust fan / supply fan / exhaust fan — is to provide some heat recovery (via the ceramic core).

The main disadvantage of the Lunos e2 is its low airflow rate. It only delivers 17.6 cfm — not much. So you’ll need several pairs of these fans to meet the needs of an average house.

Lunos fans are manufactured in Berlin, Germany by Lunos Lueftungstechnik GmbH. They are sold in the U.S. by for $1,055 per pair.

Lunos recently introduced a new model, the Lunos Nexxt — a wall-mounted HRV that uses the same type of cylindrical sleeve with a ceramic core that is used for the Lunos e2. The Lunos Nexxt ventilates at a variable rate that tops out at 53 cfm. The Nexxt is available for about $1,600 to $1,700 from 475 High-Performance Building Supply. (In some respects, the Lunos Nexxt resembles the Zehnder ComfoAir 70, a $1,250 wall-mounted ERV that ventilates at a rate up to 38 cfm. For more information on the ComfoAir 70, see Zehnder Develops a Ductless ERV.)

A less expensive knock-off of the Lunos fan is the TwinFresh Comfo fan manufactured in Kiev, Ukraine by a company called Vents. The TwinFresh Comfo is designed to operate alone — it needn’t be paired with a second unit. It is rated at 32 cfm, which is a significantly higher airflow rate than the Lunos e2. The TwinFresh Comfo is sold by Home Depot for $410.

For more information on Lunos fans, see “European Products for Building Tight Homes.”

For more information on TwinFresh Comfo fans, see “TwinFresh Comfo 32 CFM Power 5 in. Single-Room Energy Recovery Ventilator.”

An exhaust-only ventilation system. The typical exhaust-only ventilation system uses a high-quality bathroom exhaust fan (for example, a Panasonic fan). The fan either runs continuously or is controlled by a 24-hour timer for intermittent operation. (Other control options are also possible.)

Most ventilation experts discourage the use of exhaust-only ventilation systems, because these systems do a relatively poor job of distributing fresh air. (With an exhaust-only system, fresh air enters the house through random cracks in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. That means that homeowners can never be sure which rooms will get the most fresh air.) For more information on this issue, see "Ensuring Fresh Air in Bedrooms."

It must be admitted, though, that exhaust-only ventilation systems have certain positive attributes: they are inexpensive to install and use less electricity than an HRV or ERV. An exhaust-only ventilation system can be a good solution for a small house with an open floor plan.

In a recent email discussing ventilation strategies, Joe Nagan, a Wisconsin energy consultant, informed me, "From its inception in 1999, our Wisconsin New Homes Program has always required a whole-house ventilation system in addition to spot ventilation for kitchen and all rooms with a tub, toilet or shower. We did this long before most other programs. ... We test every system. ... Over the course of our program the dominant whole-house ventilation strategy has been exhaust-only. ... We rarely get a complaint from builders or homeowners, and the Energy Center of Wisconsin report showed that these homes look just fine in real-world operation. ... This is why I look favorably on exhaust-only, as long as there are no safety or soil gas concerns."

For more information on exhaust-only ventilation systems, see “Designing a Good Ventilation System.”

A CERV or Minotair unit. Two North American manufacturers (Build Equinox and Minotair) sell ventilation appliances that incorporate a small air-to-air heat pump. In the winter, the heat pump transfers heat from the exhaust air stream to the supply air stream. In the summer, the heat pump transfers heat from the supply air stream to the exhaust air stream.

Build Equinox manufactures a unit called the , or CERV.

Minotair manufactures a unit called the .

In effect, these units act like an HRV, while providing a little bit of space heating capacity in winter, and a little bit of cooling capacity in the summer. Homeowners who are concerned about future maintenance costs should be aware that the CERV and Minotair units are much more mechanically complicated than an HRV or ERV.

Most users report that the CERV and Minotair units are efficient and effective. Their main drawback: they are somewhat expensive to install. The installed cost of a CERV is about $5,000, while the installed cost of a Minotair is about $6,500 or $7,000.

For more information on the CERV, see “A Balanced Ventilation System With a Built-In Heat Pump.”

For more information on the Minotair Boreal appliance, see “Another North American Magic Box.”

Commission your system

Some types of ventilation systems — for example, Lunos fans — don't need to be commissioned. But any type of ventilation system that includes ductwork benefits from commissioning.

You can't commission a ventilation system unless you have some method of measuring airflow rates.

For more information on commissioning a ventilation system, see these articles:

Martin Holladay’s previous blog: “Kitchen Design.”

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Image Credits:

  1. Lunos and Soler & Palau

Nov 17, 2017 10:01 AM ET

Missed one.
by Dana Dorsett

Among the "Lunos-type fans" let's not forget the Lunos Nexxt, which can deliver 53cfm per pair at the full 20 watt highest speed. (~$1600/pair through 475 High Performance Building Supply.)

Nov 17, 2017 10:28 AM ET

Edited Nov 18, 2017 9:45 AM ET.

Mini ERV Options
by Armando Cobo

We install in all our houses Honeywell, Fantech and Panasonic ERVs, with capacities from 40-120cfm for $1000-1500. It covers pretty much all houses.
Revised: This is an installed price.

Nov 17, 2017 10:47 AM ET

Edited Nov 17, 2017 10:50 AM ET.

by Ethan T ; Climate Zone 5A ; ~6000HDD

My understanding is that the Minoair Boreal has been replaced by the CATU-V12 (). The cooling capacity has been upgraded; I'm not entirely sure what else has changed... The website is only partially updated, so if you click on the Canadian flag () you'll still see the Boreal - or maybe the Boreal is still the preferred model in Canada - that I'm not 100% sure about it because it seems the changeover is happening right now...

In a well insulated house with perhaps another source of heat (wood), the Minotair (especially with the resistance heat booster) can be used as an all-in-one heating, cooling, venitilation, and dehumidification solution, which should be factored into the installed cost calculations.

Nov 17, 2017 11:43 AM ET

What changed from 2009?
by Reid Baldwin

I looked back at your 2009 article and I didn't notice major changes since then. As in 2009, building scientist recognize ventilation as important, but most homeowners still don't. Building scientist still argue about the appropriate rate. The vast majority of builders and contractors still look for the cheapest way to satisfy the code inspector. Inclusion of a ventilation requirement in the model codes may have resulted in a ventilation requirement appearing in more local codes than in 2009. But inspectors still seem to accept a fresh air inlet with no controls and no commissioning as satisfying the requirement.

Nov 18, 2017 5:31 AM ET

Response to Dana Dorsett (Comment #1)
by Martin Holladay

Thanks for alerting me to the Lunos Nexxt. I have edited my article in include information on the Nexxt.

It could be argued that the Nexxt resembles the Zehnder ComfoAir 70 more than it resembles the original Lunos fans -- falling into a category that might be called "Low-airflow-rate wall-mounted ventilation appliances."

Nov 18, 2017 5:35 AM ET

Response to Armando Cobo (Comment #2)
by Martin Holladay

The airflow rates you mention -- 40 cfm to 120 cfm -- will indeed cover most U.S. homes. While it's possible to buy an ERV for $1,500, as you point out, installing the ventilation system will usually add several thousand dollars to the final cost for the homeowner.

Nov 18, 2017 5:43 AM ET

Response to Ethan T (Comment #3)
by Martin Holladay

Thanks for your comments. As I pointed out in my article on the Montair, the heat pump provides only 9,400 Btuh at an outdoor temperature of 47 degrees F -- and even less heat output as the outdoor temperature drops. That means that it's unlikely to be suitable as a whole-house heating system for a cold-climate home.

Nov 18, 2017 5:50 AM ET

Edited Nov 18, 2017 7:40 AM ET.

Response to Reid Baldwin
by Martin Holladay

Since I wrote my 2009 article, the model building codes have changed, and many builders are now required to install residential ventilation systems. As you point out, however, builders' ventilation system knowledge and installation skills fall woefully short, and the vast majority of residential ventilation systems installed by builders simply don't work. For an in-depth discussion of this issue, see my article, Ventilation Failures and Vocabulary Lessons.

The weakness in new code requirements for ventilation, as your correctly point out, is that (a) the code doesn't tell builders what they need to install to meet code requirements, so that a system that amounts to a hole in the wall (with a duct connecting the hole in the wall to a furnace plenum) meets code requirements, and (b) most code officials don't know much about ventilation and are unlikely to understand whether a system meets code requirements.

Nov 18, 2017 9:49 AM ET

Response to Martin
by Armando Cobo

The prices for the ERVs I gave above are installed prices, and with metal ducts. I fact, after my post I talked to the HVAC contractor we use in all my builder's jobs, and he said that a Fantch model we've used is 150 cfm,

Nov 20, 2017 4:32 PM ET

Supplying fresh air to bedrooms
by Adam W

@Martin -
When supplying fresh air to bedrooms - have you heard of complaints on very hot or very cold days about hot/cold air blowing directly into the bedrooms?

Nov 20, 2017 4:44 PM ET

Response to Adam W
by Martin Holladay

Q. "When supplying fresh air to bedrooms - have you heard of complaints on very hot or very cold days about hot/cold air blowing directly into the bedrooms?"

A. Yes -- specifically, about cold air in very cold weather. The complaints apply to poorly located passive air inlets -- I'm not a fan of passive air inlets in any case -- as well as poorly located registers that deliver air from an HRV.

Obviously, you don't want the supply air register to aim the airflow at the head of the bed. You want to locate the register somewhere far from the head of the bed. Some builders locate the fresh air register in the closet, and equip the closet with louvered doors.

Nov 20, 2017 4:48 PM ET

Response to Armando Cobo (Comment #9)
by Martin Holladay

Congratulations -- you're getting great prices from you ventilation system contractor if you are getting your ERVs installed for $1,500. In many parts of the U.S., that price is unrealistic.

Talk to owners of Zehnder HRVs, and you'll learn that they paid $7,000 to $10,000. (Obviously, Zehnder equipment is more expensive that the equipment you install -- but still, you're getting good pricing.)

Nov 20, 2017 4:51 PM ET

Response to Martin (comment #11)
by Adam W


My scenario would be using an ERV in Northern Virginia. I'm guessing your "very cold weather" is more likely to apply to the Northeast?

Nov 20, 2017 5:07 PM ET

Edited Nov 20, 2017 5:16 PM ET.

Response to Adam W (Comment #13)
by Martin Holladay

I think there is less likelihood of complaints when the outdoor temperature is 20 degrees F than when the outdoor temperature is -10 degrees F. That said, think ahead about where you want your fresh air registers to be installed. Don't direct the fresh air flow at the head of your bed.

Dec 2, 2017 10:03 PM ET

Edited Dec 2, 2017 10:03 PM ET.

An other alternative for whole house and wet room venting?
by Mai Tai

I have been looking for an HRV specifically designed to provide dedicated, on demand wet room venting that rivals a dedicated bath fan. So far I have only found one system that shows promise, because it actually has flappers that will close when a boost fan is triggered to isolate the wet room and provide maximum local flow:

If anyone knows of competing systems with similar features, I would be very interested to hear about them.

Dec 3, 2017 6:38 AM ET

Edited Dec 3, 2017 6:42 AM ET.

Response to Mai Tai
by Martin Holladay

Mai Tai,
The Aldes system you linked to is the only one with the features you're talking about, although plenty of HRVs can be controlled from a bathroom (with a "boost" switch to increase the ventilation rate when needed).

For more on this issue, see Does a Home with an HRV Also Need Bath Fans?

Dec 3, 2017 8:50 AM ET

Response to Mai Tai
by stephen sheehy

We've found that the boost switch on our Zehnder hrv works fine to clear moisture from the bathroom after showers. It can be set to run for 10, 30, or 60 minutes. Usually, 10 is enough.
If you are unconvinced, why not just install a separate bathroom fan?

Dec 3, 2017 9:09 AM ET

Stephen, I'm trying to avoid
by Mai Tai


I'm trying to avoid additional penetrations in the insulated shell, so no dedicated fans are planned. The issue with most standard HRVs is that once you press the boost button, it boosts flow at every inlet in the house. Some mitigate this by having multiple inlets at the main box, with only a few connected to the boost fan (I believe Zehnder is one of those). This works, but tends to complicate installation and increase tubing/duct costs. The Aldes is the only system that I have found to combine standard ducting and prioritized flow at the boost site.

Dec 3, 2017 10:27 AM ET

Reply to Mai Tai
by stephen sheehy

I understand your reluctance to making another hole in the building envelope. I've never heard of the Aldes hrv, but if it has decent specs, install one. But I think you'll be fine with a typical hrv and a boost, even if the boost isn't limited to a single location. We use the boost to clean out the kitchen air if we're cooking something smelly or smoky.

Dec 6, 2017 12:51 PM ET

Actual ventilation rates from balanced and unbalanced
by John Proctor

Martin, you may have mentioned and I didn't notice the difference between unbalanced (exhaust only or supply only) and balanced systems. Balanced systems moving the same CFM through the fans provide approximately twice the actual ventilation as unbalanced systems of the same rated CFM.

Dec 6, 2017 1:06 PM ET

Response to John Proctor
by Martin Holladay

I think you're referring to the Point Five Rule. Here's what I wrote about the Point Five Rule in an "Editor's Note" inserted into a 2013 blog by Reid Baldwin:

"...The Point Five Rule was first stated in 1992 by Larry Palmiter and Tami Bond. The Point Five Rule states that the use of an exhaust fan or a supply fan that moves less than twice the natural infiltration rate will cause an increase in airflow equal to half the flow rate of the fan. This phenomenon occurs because operation of an exhaust fan raises a house's neutral pressure plane, causing some of the wall leaks that were outlets to become inlets. Similarly, operation of a supply fan lowers a house's neutral pressure plane, causing some of the wall leaks that were inlets to become outlets."

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